Ethan Albrecht, U.S. Air Force Academy; Erik Jensen, U.S. Air Force Academy; Kody Wilson, U.S. Air Force Academy; Joshua Key, U.S. Air Force Academy; Francis Chun, U.S. Air Force Academy; Nikola Ruby, Murray State University; David Strong, Strong EO Imaging, Inc.; Casey Schuetz-Christy, Millennium Engineering & Integration Co.
Keywords: Falcon Telescope Network, Photometry, Spectroscopy, GEO Satellites, Space Situational Awareness, Space Domain Awareness
Abstract:
Ground-based, optical observations of satellites are a key component of Space Domain Awareness and support the mission of the United States Space Force. Simultaneous satellite observations from geographically diverse sites can more effectively characterize elements of the satellites orbit and optical properties. However, satellite observations cannot be quantitatively compared without a common basis for comparison. The Falcon Telescope Network (FTN) consists of 0.5-meter, f/8 Officina Stellare ProRC-500 telescopes, six in Colorado, one in Pennsylvania, one in Chile, one in Germany, and two in Australia. These telescope systems have identical hardware components such as the Software Bisque Paramount ME2, an Andor Apogee F47 camera, and an Apogee AFW50-9R filter wheel. The filter wheel contains Johnson-Cousin photometric filters (B, V, R) and a 100-lines per-millimeter Richardson diffraction grating for low-resolution slitless spectroscopy. This paper describes the process and approach we used to photometrically and spectrally calibrate the U.S.-based telescopes in the FTN using standard astronomical techniques and accepted photometric calibration stars, and stars with well-known absorption and emission lines for spectral calibration. In this study, we report on the effort to calibrate the photometric and spectral response of several of the U.S.-based FTN telescopes. The photometric calibration stars were selected from the Landolt and Oja star catalogs and were observed throughout the year. From these calibration stars, the extinction coefficients and zero points of each filter were determined for individual FTN telescopes. Of the seven U.S. FTN telescopes, we were successfully able to observe calibration stars with 6 of the telescopes across 63 nights, resulting in over 10,000 raw images. Those telescopes were located at Otero Junior College (OJC-Falcon), Grand Mesa Observatory associated with Colorado Mesa University (CMU-Falcon), Fort Lewis College (FLC-Falcon), Northeastern Junior College (NJC Falcon), and the Applied Research Lab at Pennsylvania State University (PSU-Falcon). Additionally, we calibrated the 16-inch telescope on the campus of the Air Force Academy (USAFA-16). The spectral calibration of the diffraction grating in an FTN telescope used several stars with well-known absorption lines or in the case of Wolf-Rayet stars, well-known emission lines, which allowed us to determine the pixel-to-wavelength conversion for a telescope. Of the five Falcon telescopes that were photometrically calibrated, we were only able to spectrally calibrate two telescopes, CMU-Falcon and NJC-Falcon for this study. The calibration data was collected across 2020-2021 and although only a third of the data has been processed – 20 of 63 nights – that data showed that there are diminishing returns in the value of collecting calibration data on more than 6 stars for a given night. This is important as future observation plans can be adjusted accordingly as current FTN observation plans often have 50% of the telescope time devoted to collecting calibration data while the other 50% of the time is devoted to collecting target data. It can be clearly seen that additional time can be used to collect on targets as opposed to calibration stars. The spectral calibration slope and offsets for the CMU-Falcon and NJC Falcon provide an accurate means to convert from pixel space to spectral space. Additionally, the spectral calibration slopes, offsets, and confidence intervals of the CMU-Falcon and NJC-Falcon indicates that the conversion for each telescope in the FTN are extremely similar consistent with the fact that every Falcon telescope has identical diffraction gratings. This may mean in a large network of identical telescopes, a good conversion for the population of telescopes could be obtained by characterizing a small number of telescopes.
Date of Conference: September 14-17, 2021
Track: Non-Resolved Object Characterization